Wireless communication systems may include transmitters and receivers (or combinations thereof) of wireless signals. In the case of transmission of the wireless signals, a transmitter may modulate data signals to be transmitted on to a carrier wave and a receiver may receive the modulated signals and demodulate the data signals. Various types of modulation techniques are known in the art, such as phase modulation, amplitude modulation, etc. In this disclosure, phase modulation is considered in more detail.
Phase modulation refers to a type of modulation where data signals (or information) are digitally encoded as variations in an instantaneous phase of the carrier wave. In the context of digital signal transmission, phase modulation is seen to switch between different phases. Thus, phase modulation is generally referred to as phase shift keying (PSK). Numerous types of PSK are known in the art, such as, quadrature PSK (QPSK), offset-QPSK (O-QPSK), binary PSK (BPSK), minimum-shift keying (MSK), etc. It is possible to switch between different types of PSK.
For example, considering a time-domain implementation of a QPSK modulator, an input bit stream of the data signals to be transmitted is split into in-phase (I) and quadrature (Q) waveforms, which are then separately modulated by two carriers which are in phase quadrature (e.g., a sine and a cosine carrier wave which are varied in phase, while keeping amplitude and frequency constant). This allows transmission of two bits in each modulation symbol, with four possible different symbols since the phase of the carrier wave can take on four possible values (e.g., 0, π/2, π, 3π/2), wherein each phase corresponds to a different symbol. It is seen that each modulated signal in QPSK can be represented as a BPSK signal and summed up to produce the QPSK signal. In another example, while it is possible to generate an O-QPSK waveform in a similar manner as described above for QPSK, by generating the I and Q waveforms separately using I and Q modulators, in the time-domain, O-QPSK modulation can also be achieved by generating time-domain baseband I and Q waveforms according to QPSK signaling followed by using half-sine (HS) shaping filters, and shifting the Q waveform by half a symbol period with respect to the I waveform. As yet another example, an MSK modulator can be implemented by recognizing that the difference between O-QPSK and MSK lies in the way the input bits are mapped.
Accordingly it is seen that for various types of PSK signaling in time-domain, a transmitter can be implemented using I and Q modulators. Doing so makes it possible for the modulated signals to satisfy a “spectral mask”, which defines a power spectrum according to wireless communication standards or regulations. For example, the spectral mask may be satisfied by implementing a pair of digital low-pass filters designed to suppress side-lobes of the I and Q modulated signals in their signal power spectrum. In time-domain, finding filter coefficients that achieve both satisfactory side-lobe suppression and satisfactory error vector magnitude (EVM) is relatively straightforward, and therefore, transmitters which implement time-domain PSK signaling can be designed to meet the spectral mask and EVM using conventional approaches known in the art.
With the exploration of low-cost RF communication (e.g., WiFi, Bluetooth, Bluetooth Low Energy (BLE), etc.) seen in recent times, for example, in emerging markets such as Internet-of-Things (IoT), frequency-domain PSK signaling is recognized as a better alternative to time-domain PSK signaling, since implementations of transmitters using frequency-domain signaling can incur less costs in comparison to transmitters using time-domain signaling.
For example, O-QPSK and MSK signals can also be generated using a frequency synthesizer, rather than the separate I and Q modulators as discussed above in the time-domain. In the frequency-domain, O-QPSK and MSK modulation can involve mapping the data signals or information bits to corresponding waveforms in the frequency domain and feeding the mapped symbols to a frequency synthesizer which generates a frequency modulated (FM) signal. A modulator using the frequency synthesizer can be designed with less area and can consume less power in comparison to the I and Q modulators. However, unlike the straightforward case of I and Q modulators in the time-domain, the spectral mask and EVM requirements in the frequency domain are more difficult to satisfy, as these metrics are related to standards specified for the low-cost applications discussed above (e.g., the well-known IEEE 802.15.4 standard). At high transmit power levels (which can be desirable in many scenarios in this application space), conventional frequency-domain implementations of O-QPSK and MSK modulation, for example, with analog frequency synthesizers, are seen to violate or exceed the specified spectral mask in an attempt to meet the EVM requirements.
Thus, there is recognized a need in the art for designs of transmitters in the frequency domain which can meet spectral mask and EVM requirements for various modulation schemes such as O-QPSK or MSK in the frequency domain, while retaining the desirable characteristics of low cost and low power.